make pruning of CRDT garbage work, #21647

* fix merge issues of DataEnvelope and its pruning
* simplify by removing the tombstones, which didn't work in all cases anyway
* keep the PruningPerformed markers in the DataEnvelope until configured
  TTL has elapsed (wall clock)
* simplify PruningState structure
* also store the pruning markers in durable data
* collect removed nodes from the data, listing on MemberRemoved is not enough
* possibility to disable pruning altogether
* documented caveat for durable data

akka-distributed-data/src/main/scala/akka/cluster/ddata/ORMultiMap.scala

@@ -204,6 +204,9 @@ final class ORMultiMap[A, B] private[akka] (private[akka] val underlying: ORMap[
     else
       this
 
+  override def modifiedByNodes: Set[UniqueAddress] =
+    underlying.modifiedByNodes
+
   override def needPruningFrom(removedNode: UniqueAddress): Boolean =
     underlying.needPruningFrom(removedNode)